❌ Wrong! Change in colour IS a sign of a chemical reaction — for example, iron nails turn reddish-brown when copper sulphate reacts with iron.

✅ Correct!  +1 mark

A container merely changing its shape is a physical change. No new substance is formed, so it is not a sign of a chemical reaction.

❌ Wrong! Gas evolution is a definite sign of a chemical reaction — e.g., hydrogen gas bubbles up when zinc reacts with dilute sulphuric acid.

❌ Wrong! Temperature change (heat given out or absorbed) is a key sign of a chemical reaction — e.g., mixing quick lime with water produces heat.

❌ Wrong! Balancing an equation satisfies the law of conservation of mass, not energy.

✅ Correct!  +1 mark

The total number of atoms of each element must be equal on both sides — satisfying the law of conservation of mass (mass cannot be created or destroyed).

❌ Wrong! The law of definite proportions relates to the fixed ratio of elements in a compound, not to balancing equations.

❌ Wrong! The law of multiple proportions deals with different compounds formed by the same elements, not equation balancing.

❌ Wrong! CaO + H₂O produces a single product Ca(OH)₂ — that is combination, not decomposition. Also, heat is released, making it exothermic.

✅ Correct!  +1 mark

CaO + H₂O → Ca(OH)₂ + Heat. Two reactants form one product (combination). Heat is released (exothermic). The beaker becomes warm to the touch.

❌ Wrong! No element is displaced here. CaO combines with water to form calcium hydroxide — it is a combination reaction.

❌ Wrong! The reaction IS a combination reaction, but it releases heat (exothermic), not absorbs it.

❌ Wrong! The gaseous state is shown by (g). Examples: H₂(g), CO₂(g).

✅ Correct!  +1 mark

(aq) stands for aqueous — meaning the substance is dissolved in water. Example: NaCl(aq), CuSO₄(aq). The other symbols are (s)=solid, (l)=liquid, (g)=gas.

❌ Wrong! Pure liquid state is shown by (l), like H₂O(l). (aq) specifically means dissolved in water.

❌ Wrong! Solid state is shown by (s), like Fe(s) or CaCO₃(s).

❌ Wrong! In combination, two or more reactants form one product. Here one reactant breaks into three products — the opposite.

✅ Correct!  +1 mark

Lead nitrate breaks down into lead oxide, nitrogen dioxide and oxygen when heated. A single substance decomposes using heat — this is thermal decomposition.

❌ Wrong! In displacement, one element pushes another out of a compound. Here only one reactant breaks apart — no displacement occurs.

❌ Wrong! Double displacement needs two reactants exchanging ions. This has only one reactant decomposing with heat.

❌ Wrong! Copper sulphate does not evaporate at room temperature. The colour fades because a chemical reaction is consuming it.

✅ Correct!  +1 mark

Fe + CuSO₄ → FeSO₄ + Cu. Iron is more reactive than copper, so it displaces copper. Blue CuSO₄ is replaced by pale green FeSO₄ — hence the colour fades.

❌ Wrong! Iron does not just dissolve — it chemically reacts with copper sulphate, displacing copper. The iron nail actually gets a brownish copper coating.

❌ Wrong! Water does not absorb colour. The fading is due to the chemical displacement of blue copper sulphate by the formation of iron sulphate.

❌ Wrong! Carbon + oxygen forming carbon dioxide is a combination reaction — two substances form one product.

✅ Correct!  +1 mark

Na₂SO₄ and BaCl₂ exchange their ions — SO₄²⁻ swaps with Cl⁻ to form the white precipitate BaSO₄. Ion exchange between two reactants = double displacement.

❌ Wrong! Zinc displacing copper from copper sulphate is a single displacement (not double displacement) reaction.

❌ Wrong! Calcium carbonate breaking down when heated is a thermal decomposition reaction.

❌ Wrong! Rancidity is the oxidation of fats and oils in food, causing a stale smell and taste. It does not apply to metals.

✅ Correct!  +1 mark

Corrosion is the gradual destruction of metals by reaction with moisture, acids, or other substances in the environment. The green layer on copper is basic copper carbonate formed by corrosion.

❌ Wrong! Precipitation is the formation of an insoluble solid (precipitate) during a reaction between two solutions. It is not related to the slow degradation of metals.

❌ Wrong! Sublimation is the direct change of a solid to a gas. It has nothing to do with the green coating on copper.

❌ Wrong! Reduction = loss of oxygen (or gain of hydrogen). If a substance gains oxygen, it is being oxidised, not reduced.

✅ Correct!  +1 mark

Oxidation = gain of oxygen OR loss of hydrogen. In CuO + H₂ → Cu + H₂O, hydrogen gains oxygen and is oxidised. CuO loses oxygen and is reduced.

❌ Wrong! Precipitation refers to an insoluble solid forming in a solution. It is not related to the gain or loss of oxygen.

❌ Wrong! Neutralisation is the reaction of an acid with a base. It is not related to oxidation-reduction terminology.

❌ Wrong! Nitrogen is an inert, colourless, odourless gas. It does not add flavour. It is used to prevent oxidation.

✅ Correct!  +1 mark

Fats in chips react with oxygen (oxidation) causing rancidity — an unpleasant smell and taste. Nitrogen being inert displaces oxygen, preventing this oxidation reaction.

❌ Wrong! While nitrogen is dry, its primary purpose is to prevent oxidation of fats and oils that causes rancidity, not just to control moisture.

❌ Wrong! Sealing is a mechanical process unrelated to the gas inside. Nitrogen is chosen specifically to prevent oxidation of the fats.

❌ Wrong choice. Respiration is EXOTHERMIC. Glucose combines with oxygen to release energy (heat) for all life processes: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy.

✅ CORRECT!  +1 mark

Respiration is EXOTHERMIC — energy is released, not absorbed. The breakdown of glucose provides energy to the body. Endothermic reactions absorb energy from surroundings.

✅ CORRECT!  +1 mark

2H₂O → 2H₂ + O₂. When electric current passes through water (electrolytic decomposition), hydrogen gas collects at the cathode and oxygen at the anode. The volume of H₂ is double that of O₂.

❌ Wrong choice. This is TRUE. Electrolysis of water decomposes it into hydrogen (at cathode) and oxygen (at anode). The H₂ volume is always twice the O₂ volume.

❌ Wrong choice. A skeletal equation may NOT be balanced. Example: Mg + O₂ → MgO is skeletal but unbalanced (1 O on RHS vs 2 O on LHS). It needs to be balanced using coefficients.

✅ CORRECT!  +1 mark

A skeletal equation just shows the correct formulae of reactants and products without balancing. The atoms are typically NOT equal — that is why we need to balance it using the hit-and-trial method.

❌ Wrong choice. Zinc IS MORE reactive than copper. Zn + CuSO₄ → ZnSO₄ + Cu. A more reactive metal can always displace a less reactive metal from its salt solution.

✅ CORRECT!  +1 mark

FALSE — Zinc is MORE reactive than copper. Zn + CuSO₄ → ZnSO₄ + Cu. A more reactive metal displaces a less reactive metal. Zinc CAN and DOES displace copper from copper sulphate solution.

✅ CORRECT!  +1 mark

Rancidity is caused by the oxidation of fats and oils. Airtight containers prevent oxygen from reaching the food, slowing down the oxidation reaction and keeping the food fresh longer.

❌ Wrong choice. This is TRUE. Rancidity is oxidation of fats. Airtight containers limit oxygen exposure, which directly slows the oxidation process causing rancidity.

✅ CORRECT!  +1 mark

Silver chloride (AgCl) and silver bromide (AgBr) decompose when exposed to sunlight, forming silver metal and releasing the halogen. The silver deposits create the dark areas in black and white photographic film.

❌ Wrong choice. This is TRUE. 2AgCl → 2Ag + Cl₂ (sunlight). Silver halides on photographic film decompose in light to deposit silver metal, forming images. This photolytic decomposition is the basis of photography.

✅ COMBINATION REACTION

Two or more reactants join to form a single product. Example: CaO + H₂O → Ca(OH)₂. The number of products is always fewer than the number of reactants.

✅ DECOMPOSITION REACTION

A single substance breaks into simpler products when energy (heat/light/electricity) is supplied. Example: 2FeSO₄ → Fe₂O₃ + SO₂ + SO₃ (thermal decomposition).

✅ REDOX (Oxidation-Reduction) REACTION

CuO loses oxygen (reduced → Cu) while H₂ gains oxygen (oxidised → H₂O). In every redox reaction, one substance is oxidised while the other is simultaneously reduced.